1) Field of the Invention
The present invention relates to a semiconductor laser modulation driving apparatus that drives modulation of an optical output of a semiconductor laser (laser diode (LD)) serving as a light source in an image forming apparatus such as a laser printer, an optical disk apparatus, a digital copying machine, an optical communication apparatus, or the like that forms an image by modulating the optical output of the light source, and an image forming apparatus that includes the semiconductor laser modulation driving apparatus.
2) Description of the Related Art
Some of the well-known methods for modulating an optical output of a semiconductor laser light source are, for example, a power modulating method for modulating a light quantity itself, a pulse width modulating method for modulating a lighting-up time, and a power-pulse width modulating method that is a combination of the power modulating method and the pulse width modulating method. One proposed method of a pulse width modulating method includes generating a chopping wave or a sawtooth wave corresponding to each pulse generation cycle, comparing each wave with an analog video signal using a comparator, and thereby generating a pulse width modulated signal. Another method of a pulse width modulating method includes generating a high frequency clock, generating a delay pulse by digitally dividing the clock, and generating a pulse width modulated signal by an OR operation or an AND operation.
A conventional apparatus for forming an image by thus modulating the optical output of the semiconductor laser light source is shown in FIG. 3. Such conventional apparatus normally includes an image data generation unit 110, an LD modulation signal generation unit 120, and an LD driving unit 170. Based on a gamma characteristic or the like of a photoconductor, the image data generation unit 110 converts data output by a printer, data read by a scanner, or the other data, into image data that can be output by a laser printer or the like. The LD modulation signal generation unit 120 generates an LD modulation signal so that the image data can be subjected to power modulation or pulse width modulation in a semiconductor laser (LD) 190. The LD driving unit 170 drives the LD 190 in response to the LD modulation signal. As shown in FIG. 3, each of the image data generation unit 110, the LD modulation signal generation unit 120, and the LD driving unit 170 is formed on a printed circuit board (PCB), or incorporated into an application specific integrated circuit (ASIC).
If an image signal transfer rate is compared with an LD modulation signal transfer rate, the LD modulation signal transfer rate is faster. This is why it is preferable that in the image forming apparatus, the LD modulation signal generation unit 120 and the LD driving unit 170 are arranged as close as possible. For example, Japanese Patent Application No. 3283256 discloses the image forming apparatus in which, a pulse width modulation circuit, a recording element, and a driving circuit are formed on a singe circuit board. A digital image control circuit is formed on a circuit board different from this single circuit board. In addition, differential digital image signals are transmitted to the pulse width modulation circuit.
However, demand for further accelerating an operating rate of the image forming apparatus is on the rise. To meet this demand, the number of LDs to be driven increases, so that rather than two or four, eight LDs or LD arrays are used to form a monochrome image. Further, because the copying machines or printers are gradually shifting from monochrome to colored ones, a plurality of LD modulation signal generation units and LD driving units are required, accordingly. For example, if image data has eight bits, and an LD modulation signal has one bit per color, and if four colors are driven by two LDs, then the image data has 64 bits, and the LD modulation signal has eight bits in all. In the example of the configuration of the conventional image forming apparatus shown in FIG. 3, it is necessary to transfer 64 signals at a high rate although those signals are not as fast as the LD modulation signal. As a result, the number of data lines increases, a configuration of the transfer unit is greatly complicated, and data is more difficult to be transferred at the high rate. Besides, if differential digital image signals supplied from the image data forming unit such as the digital image control circuit to the LD modulation signal generation unit such as the pulse width modulation circuit are transferred, not only the configuration of the transfer unit but also a configuration of transmission lines are further complicated.